In a modern paper mill, the paper making process is monitored continuously. Paper is produced from pulp slurry as a continuous web that is reeled at the end of the process. There are several measuring points in the process for monitoring the process equipment as well as the raw material, pulp in the wet end of the paper machine and the end product, that is, paper in the dry end of the machine.
Paper measurements are made both in the paper laboratory of the mill and on-line, during the paper making process, when the paper is moving continuously in the machine. The measuring unit in the paper machine is usually set up in an open draw of the travelling path of the web, consisting of a measuring beam extending across the web, in its CD-direction (Cross Direction). In the measuring beam, there are mounted two sensor platforms, one adjacent to each of the web surfaces, where the measuring sensors are located so that the two heads of each sensor are facing each other and located on the opposite sides of the web. The platforms traverse the web back and fourth in its cross direction, while the web is moving. In a modern paper machine, the paper travels in a speed of 1600-2200 meters/minute. The sensor mounted on a platform is moving across the web in a speed of about 30-60 meters/minute. So the sensors travel across the web along a zig-zag travelling path. Also measuring units, set up in such a way that only certain position in the CD-direction of the web is measured, are being used. In this arrangement, the sensor platforms are set up at a fixed point in the CD-direction of the web.
Optical properties of paper, such as opacity, color, whiteness, brightness and fluorescence index are measured by illuminating the web and detecting the light reflected from and optionally also transmitted through the paper. In many paper products, like books or newspapers, the user sees a stack of multiple paper sheets rather than a single layer of paper. If paper is not totally opaque, color and other optical properties of the stack are different than those of a single sheet. This is because some of the light reaching the observer's eye is reflected from the sheets placed below the top one. Typically, at a paper mill's laboratory, measurements of optical properties of paper are made against a backing of an opaque stack of the same paper of multiple paper sheets. This is to eliminate the effect of the incomplete opacity of a single sheet on the measurements. When measuring a continuously moving paper web on-line, it is not possible to form a stack of paper sheets as required and thus the method used at laboratory cannot be used. Close match between on-line measurements and off-line measurements, e.g. laboratory measurement results is a basic requirement in nowadays papermaking. Thus, the on-line measurement needs to have means for compensation of the effects of incomplete opacity on the results.
Several methods for measuring opacity compensated paper web color on-line have been suggested. These methods measure the single thickness of the web and aim at producing a measuring result that is comparable with the laboratory measurements of a stack of paper. One possibility is to measure the reflectance of the web against a suitable opaque backing resembling the color and reflectivity of the measured paper. An effect of the backing on the measured color is similar to that of a stack of paper. However, this method is not sufficient: the properties of the backing are never the same as that of the measured web. Moreover, the change of paper color requires a change of backing.
Another method is to use two essentially different backings for the measurement, for example one of them being a black backing, which is highly absorptive and the other being a white backing that is highly reflective. The reflectance of the paper against both of these backings is measured. From these measurements it is possible, for example by using the Kubelka-Munk theory, to calculate the influence of the sheet transmittance on the reflectance of the paper stack and deduce the reflectance both for a single sheet and a stack of paper sheets. The problem with this measurement is that the measurements against different backings are made at different times, one after the other, because the method requires the changing of the backings between the measurements. This change can only be done by relatively slow mechanical means. As the web and possibly also the sensor is moving, these measurements against white and black backings are made from different parts of the paper, and this decreases the usability, quality and speed of the color measurement. Another problem with this arrangement is that the sensor construction includes moving parts for changing the backings that need maintenance.
A modification of this method is presented in U.S. Pat. No. 4,944,594, where instead of using two different backings, an optical gating means is placed adjacent the paper to provide a backing for the reflectance measurement. The optical gating has two operating states, a dark state and a bright state. When switched to dark state, the optical gating absorbs substantially all of the transmitted radiation and when switched to bright state, the optical gating reflects substantially all transmitted radiation back to the sheet. This solution addresses the problem of measurement against different backings taking place in different areas of the web, but brings out other challenges, mainly in the form having simultaneously sufficient reflectivity and enough contrast between the two states, as well as the stability of the states.
Yet another method is presented in U.S. Pat. No. 5,793,486, where it is suggested to measure the white and black backings simultaneously. The measurement is done by using two spectrometers, where the first spectrometer measures the radiation reflecting from the paper upon the black backing and the second spectrometer measures the radiation reflecting from the paper upon the white backing. This solution also addresses the problem of measurement against different backings taking place in different areas of the web, but brings out other challenges: this method requires at least partially different optical paths for illuminating the web upon white and black backing and requires at least partially different optical paths for detecting the light from the web upon white and black backing and it requires two separate spectrometers. The complexity and cost of the measurement increases and for example the temperature stabilization becomes more challenging in harsh papermaking environment.
Also it is known to provide opacity compensated measurement of color of a moving web, by measuring the reflectance of the web over an opaque backing with specified reflectance properties and the transmittance of the web for opacity correction. This measuring principle is schematically shown in FIG. 1. A web 2, that travels in a machine direction indicated by an arrow A, has a first surface 2a and a second surface 2b. An on-line color measuring apparatus 1 includes a first sensor head 3 which is disposed adjacent the paper web surface 2a, above the web 2 and a second sensor head 4, which is disposed in close proximity of the paper web surface 2b, under the web. In sensor head 3 a light source 5 is arranged to illuminate the web surface 2a through a window 6. The light source 5 can be either a continuous lamp with or without a chopper or a flashing lamp. In sensor head 3, there is also a detector 7, such as a spectrometer for detecting the radiation entering the sensor head 3 through the measuring window 6. The second sensor head 4 is placed on the other side of the web, in fixed position relative to the sensor head 3. In sensor head 4, in close proximity of the web there is disposed a means 8 for providing an opaque backing for measuring the reflectance of the web 2. One example of a suitable backing means 8 is shown in FIG. 1a as an upper view. The backing means 8 is a round disc in which is arranged at least one opaque backing element 8a and at least one measurement window 8b having sufficient transmittance properties for transmittance measurements. Also an additional measurement window 8c having different transmittance properties and another opaque backing element 8d having different reflectance properties can be used in the measurement. Usually the opaque backing elements 8a, 8d are white or black in color and their transmittance is 0%. For transmittance measurement the opaque backing element is moved out of the illumination path and replaced with one of the measurement windows 8b, 8c by rotating the means 8. The means 8 can be for example a spinning wheel with at least an opaque backing area and a transparent area for transmittance measurement. For rotating and positioning the means 8 as required, there are electro-mechanical means in the sensor head 4 (not shown in the FIG. 1). When measuring the transmittance of the web, the radiation from light source 9 travels through the measuring window, the protection window 10 of the sensor head 4, the web 2 and the protection window 6 of the sensor head 4 to the detector 7. In both sensor heads 3 and 4, there are also filters and other equipment that have not been presented in FIG. 1 for clarity.
The drawback of using this measuring equipment is that a relatively slow electro-mechanical means is required to rotate and position the means 8. This means, that the reflectance and transmittance of the web are measured at different times and thus from different areas of the web. Usually the measurement is done by measuring reflectance and transmittance during separate scans resulting in considerable time delay between the measurements and thus poor opacity compensation.